Projection exposure apparatus

ABSTRACT

A projection exposure apparatus includes an illumination optical system for illuminating a pattern formed on a first object, with light, a projection optical system for projecting the pattern of the first object, illuminated by the illumination optical system, onto a second object for exposure of the same with the pattern, a main system including the illumination optical system and the projection optical system, and an interferometer for use in measurement of an optical characteristic of the projection optical system and being mounted on the main system.

FIELD OF THE INVENTION AND RELATED ART

[0001] This invention relates to a projection exposure apparatus forsemiconductor manufacture and, more particularly, to a projectionexposure apparatus for semiconductor manufacture which is usable in alithographic process for production of semiconductor devices or liquidcrystal display devices, for example.

[0002] The density of an integrated circuit is increasing, and thusprojection exposure apparatuses for semiconductor manufacture shouldhave a very high resolving power for projection exposure of a wafer to acircuit pattern formed on a reticle. In projection optical systems ofsuch projection exposure apparatuses, for improvements of theresolution, the numerical aperture (NA) has been enlarged or light ofshorter wavelengths have been used. At present, with a projectionexposure apparatus having a light source of KrF excimer laser (λ=248 nm)and NA of 0.6, a resolution of 0.18 micron is attainable.

[0003] Recently, super-resolution exposure techniques based on modifiedillumination such as ring-zone illumination or quadrupole illuminationhave been proposed. A resolution of 0.15-0.1 micron may be attainablewith them.

[0004] For production of a high resolution projection optical system, itis necessary to perform precise adjustment after a projection opticalsystem is assembled. More specifically, for a projection optical system,optical evaluations in regard to spherical aberration, comma,distortion, and exposure magnification, for example, should be done.While adjusting lens group spacings or eccentricities, the opticalperformance that satisfies predetermined specifications is pursued.Usually, the evaluation of optical performance is made by projecting andprinting an image of a mask pattern upon a resist (photosensitivematerial) applied to a photosensitive substrate (wafer) and byobserving, after development, a resist image formed thereon.

[0005] As an alternative method, there is a method in which wavefrontaberration of a projection optical system is measured by use of aninterferometer. However, this method requires use of a specialapparatus.

[0006] As described above, in projection exposure apparatuses, it isnecessary to check the quality of a resist image for final lensperformance adjustment of a projection optical system. However, thisprocedure involves very complicated processes such as printing a patternon a resist-coated wafer, developing the wafer, and observing a resistimage by use of a scan type electron microscope (SEM).

[0007] Additionally, since, after the optical adjustment and evaluation,a projection optical system should be mounted on a projection exposureapparatus with its lenses and lens groups held fixed so that theperformance does not change, it is very difficult to adjust a projectionexposure optical system once the projection optical system isincorporated into the projection exposure apparatus. Practically,however, in wafer exposure processes, the projection optical system isinfluenced by irradiation with illumination light and the imageperformance thereof changes thereby.

[0008] Conventional projection exposure apparatuses are not equippedwith any effective means for measuring wavefront aberration of aprojection optical system after the same is mounted on the projectionexposure apparatus. The goal for re-adjustment for image performance istherefore unfixed, and usually the operation is interrupted to suppressthe change.

SUMMARY OF THE INVENTION

[0009] It is accordingly an object of the present invention to provide aprojection exposure apparatus by which measurement of image performanceof a projection optical system, being mounted on the projection exposureapparatus, can be done easily.

[0010] These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a schematic view of a projection exposure apparatusaccording to a first embodiment of the present invention.

[0012]FIG. 2 is a schematic view of a projection exposure apparatusaccording to a second embodiment of the present invention.

[0013]FIG. 3 is a schematic view of a projection exposure apparatusaccording to a third embodiment of the present invention.

[0014]FIG. 4 is a schematic view of a projection exposure apparatusaccording to a fourth embodiment of the present invention.

[0015]FIG. 5 is a schematic view of a projection exposure apparatusaccording to a fifth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] In some preferred embodiments of the present invention to bedescribed below, a main assembly of a projection exposure apparatus isequipped with an interferometer for measurement of an opticalperformance of a projection optical system, by which wavefrontmeasurement for the projection optical system can be done directly uponthe main assembly of the projection exposure apparatus.

[0017]FIG. 1 is a schematic view of a projection exposure apparatusaccording to a first embodiment of the present invention. In thisembodiment, the invention is applied to an excimer laser stepper havingan exposure wavelength of 248 nm.

[0018] Denoted in the drawing at 1 is a KrF excimer laser which is alight source for exposure (lithography). Light emitted from the lightsource 1 enters a beam shaping optical system 2 by which it is shapedinto a beam shape being symmetrical with respect to an optical axis.Through an incoherency transforming unit 3, the coherent length of thelight is reduced. Then, the light goes through an illumination opticalsystem 4, and it illuminates a reticle 15. The reticle 15 has a desiredpattern formed thereon. The reticle pattern is then projected by aprojection optical system 16 and is imaged at a position 17. Denoted at18 is a chuck for carrying a wafer thereon. It is fixedly mounted on astage. In addition to these components, the projection exposureapparatus includes an alignment detection optical system, a focusdetection system and so on, all constituting a main system. They are notillustrated in FIG. 1, for simplification of the illustration.

[0019] Next, the structure of an interferometer for measurement of thewavefront of a projection optical system, will be described. Here, thearrangement shown in FIG. 1 is an example wherein a Fizeau typeinterferometer is provided at the reticle side.

[0020] In a case where the exposure light source comprises an excimerlaser, usually the coherent length is about several ten millimeters,whereas the total length of a projection optical system which is thesubject of measurement is about 1,000 millimeters. For this reason, itis practically unable to provide a Fizeau type interferometer. Inconsideration of it, in this embodiment, a light source separate fromthe exposure light source is used exclusively for an interferometer formeasurement of the wavefront of the projection optical system.

[0021] Denoted in the drawing at 6 is the light source to be usedexclusively with the interferometer. Since the exposure wavelength is248 nm in this embodiment, a light beam of 248 nm, corresponding to asecond harmonic of Ar laser is used. The Ar laser beam goes via a mirrorand then through a condenser system 7 and a pinhole 8. By means of acollimator lens 9, the laser beam is transformed into a parallel beam.The diameter of the pinhole 8 is set as approximately the same as anAiry disc determined by the numerical aperture of the collimator lens 9.Therefore, the light beam emitted from the pinhole 8 comprises asubstantially idealistically spherical wave. Since the collimator lens 9is designed and produced substantially free from aberration, it can beconsidered that the light emitted from the collimator lens 9 comprisesan idealistically plane wave. In the structure of FIG. 1, the light fromthe light source 1 may be guided to the pinhole 8 by use of apolarization plane preserving fiber.

[0022] The thus produced parallel beam goes via a half mirror 10 and amirror 11, and it enters a lens 12 which is what can be called a TS lens(Fizeau lens) wherein the final face functions as a reference surface.The mirror 11 and the lens 12 are held by an X-Y-Z stage 5.

[0023] Usually, steppers include reticle-to-wafer aligning means whichmay comprise a TTR alignment scope for detecting the wafer positionthrough the reticle, and such alignment scope may be mounted on and heldby a moving mechanism for moving the TTR alignment scope to a desiredposition on the reticle. In this embodiment, such TTR alignment scope isused also as the interferometer objective lens 12 described above.

[0024] The interferometer objective lens 12 should be retracted out ofthe path of exposure light of the projection optical system 16 in theexposure process, while on the other hand, it should be moved onto thelight path of the projection optical system for measurement of thewavefront aberration. When the TTR alignment scope is used as thedetection optical system as described, since the TTR alignment scope canbe moved to any desired position upon a reticle, wavefront measurementcan be done with respect to plural points on the picture field in theexposure region.

[0025] The curvature radius of the final face of the objective lens 12,at the reticle side thereof, is equal to the distance to the position 15which is equivalent to the pattern surface of the reticle. Thus,reflection light from that final face is directed, as reference light,to a light receiving surface of a CCD 28 through the mirror 11, halfmirror 10 and a condensing system 27.

[0026] On the other hand, the light beam passed through the objectivelens 12 is imaged at the position 15, corresponding to the reticlepattern position, and then it is imaged again by the projection opticalsystem 16 at a position 17 which is at the wafer side thereof. There isa spherical surface mirror 20 disposed on the stage 19, and thecurvature radius of the spherical mirror 20 is made equal to thedistance from the imaging position 17 of the projection optical system.Thus, the light reflected by the spherical mirror 20 is collected againat the imaging position 17 of the projection optical system, and it goesagain through the projection optical system, the objective lens 12, themirror 11 and the half mirror 10. The light then passes the condensingsystem 27, and it is directed to the light receiving surface of the CCD28. Since the light beam passing through the projection optical system16 interfere with the reference beam as reflected by the final face ofthe objective lens 12 as described above, the wavefront of theprojection optical system can be measured, on the basis of it. Thus, byanalyzing the outputs of the CCD 28 in a work station 50, annexed to theexposure apparatus, wavefront aberration as well as various aberrationsof the projection optical system 16 such as wavefront aberration andfield curvature, for example, causing the wavefront aberration, can bemeasured.

[0027] The spherical mirror 20 comprises a concave surface mirror inthis example. However, a spherical mirror having a convex surface mirrormay be used to provide an interferometer system. In that occasion, thecurvature center position of the convex surface mirror should beregistered with the imaging position 17, and the mirror should be placedat an opposite side as compared with the concave surface mirror. As afurther alternative, a plane surface mirror (or a wafer surface insubstitution therefor) may be used. In that occasion, with vertexreflection, only a revolutionally symmetrical component of wavefrontaberration can be detected.

[0028] Any error in relation to the wavefront which is involved in theinterferometer itself, such as the final face of the objective lens 12or the spherical mirror 20, for example, should be distinguished fromthe wavefront aberration of the projection optical system 16 to beexamined. To this end, it is necessary to measure the wavefrontbeforehand, in accordance with a system error measuring method. Thewavefront of the projection optical system 16 can be measured exactly,by correcting the wavefront error while subtracting it from themeasurement results for the projection optical system 16.

[0029] For further enhancement of measurement precision, the measurementthrough the interferometer may be performed in accordance with a fringescan method. The fringe scan can be accomplished by actuating a PZTdevice (not shown) inside the wafer stage 19 to shift the mirror 20 inthe optical axis direction by an amount of about the wavelength, toperform phase modulation of the wavefront. In this connection, movingmeans which is provided for focus adjustment of the projection exposureapparatus may be used as the moving means for moving the sphericalmirror 20 in the optical axis direction.

[0030] From the measurement of the wavefront of the projection opticalsystem, information regarding the wavefront aberration at a measurementpoint is obtainable. Further, a revolutionally symmetrical component anda revolutionally asymmetrical component of the wavefront aberration asobtained through the measurement of the wavefront of the projectionoptical system 16 as well as the X-Y-Z coordinates of the objective lens12 and the spherical mirror 20 as obtained from a measuring deviceduring the wavefront measurement may be combined with each other, bywhich distortion and field curvature, corresponding to theinterrelationship among the measurement points of the projection opticalsystem can be determined.

[0031] The field curvature of the projection optical system can bedetected by measuring the wavefront of the projection optical systemwith respect to plural points within the picture plane. Morespecifically, once the coordinate position of the detection opticalsystem of the interferometer upon the wavefront measurement, thewavefront as measured by the interferometer, and the coordinate positionof the spherical mirror 20 with respect to the optical axis direction ofthe projection optical system 16 are determined, the field curvature canbe calculated from the information related to the plural points. Thecomponent of wavefront aberration which is very important in regard tocalculation of the field curvature is the revolutionally symmetricalpower component (defocus component) of the measured wavefront.

[0032] Distortion of the projection optical system can also be detectedby measuring the wavefront of the projection optical system with respectto plural points within the picture plane. More specifically, once thecoordinate position of the detection optical system of theinterferometer upon the wavefront measurement, the wavefront as measuredby the interferometer, and the coordinate position of the sphericalmirror 20 with respect to a direction orthogonal to the optical axis ofthe projection optical system 16 are determined, distortion of theprojection optical system 16 can be calculated from the informationrelated to the plural points. The component of the wavefront aberrationwhich is very important in regard to calculation of distortion is therevolutionally asymmetrical component (tilt component) of the measuredwavefront.

[0033] On the basis of the results of measurement, a predetermined lensor lenses of the projection optical system 16 may be displaced, by whichthe aberration of the projection optical system can be adjusted andcontrolled into a desired state.

[0034]FIG. 2 is a schematic view of a second embodiment of the presentinvention. Like the first embodiment, in this embodiment the inventionis applied to an excimer laser stepper having an exposure wavelength of248 nm. In this embodiment, a Twyman-Green type interferometer isprovided on the reticle side.

[0035] Denoted at 6 is a light source for the interferometer, from whicha light beam of 248 nm corresponding to the second harmonic of Ar laseris extracted. The laser beam goes via a mirror, a condensing system 7and a pinhole 8. Through an optical system 9, it is transformed into aparallel beam. The parallel light beam is then divided by a half mirror10 into two light beams. The light beam passing through the half mirror10 is reflected by a mirror 29 as a reference beam, and the reflectedlight beam is then reflected by the half mirror 10. After beingreflected, the light beam passes through a condensing system 27 and itimpinges on a light receiving surface of a CCD 28.

[0036] On the other hand, the light beam reflected by the half mirror 10goes via a mirror 11, and it enters an objective lens 13. The light beampassing through the objective lens 13 is once imaged at a position 15corresponding to the reticle pattern position, and then it is re-imagedby the projection optical system 16 at a position 17 on the wafer side.There is a stage 19 on which a spherical surface mirror 20 is mounted.The mirror has a curvature radius which corresponds to the distance fromthe imaging position 17 of the projection optical system. Thus, thelight reflected by the spherical mirror 20 is collected again at theimaging position of the projection optical system. Then, it goes backthrough the projection optical system 16 and passes via the objectivelens 13, the mirror 11, the half mirror 10 and the condensing system 27.Finally, it impinges on the light receiving surface of the CCD 28. Thelight beam passing through the projection optical system 16 interferewith the reference beam described above, such that the wavefront of theprojection optical system can be measured.

[0037] The correction of a system error in the measured wavefront, useof a fringe scan method for enhancement of measurement precision, use ofa spherical mirror of convex surface mirror type, and calculation ofaberrations of the projection optical system may be done in a similarway as the first embodiment. On the basis of the results ofmeasurements, a predetermined lens or lenses of the projection opticalsystem 16 may be displaced, by which the aberrations of the projectionoptical system can be adjusted and controlled into a desired state.

[0038]FIG. 3 is a schematic view of a third embodiment of the presentinvention. Like the first embodiment, this embodiment is directed to anexcimer laser stepper having an exposure wavelength of 248 nm. In thisembodiment, a radial share type interferometer is provided on thereticle side.

[0039] Denoted at 6 is a light source for the interferometer, from whicha light beam of 248 nm corresponding to the second harmonic of Ar laseris extracted. The laser beam goes via a mirror, a condensing system 7and a pinhole 8. Through an optical system 9, it is transformed into aparallel beam. The parallel light beam is then reflected by a halfmirror 10, and it is directed via a mirror 11 to an objective lens 13.The light beam passing through the objective lens 13 is imaged at aposition 15 corresponding to the reticle pattern position, and then itis imaged again by the projection optical system 16 at a position 17 onthe wafer side. There is a stage 19 on which a spherical surface mirror20 is mounted. The spherical mirror 20 has a curvature radius whichcorresponds to the distance from the imaging position 17 of theprojection optical system. Thus, the light reflected by the sphericalmirror 20 is collected again at the imaging position 17 of theprojection optical system, and it goes back through the projectionoptical system. Then, it advances via the objective lens 13, the mirror11 and the half mirror 10, and it is introduced into an interferometerhaving components denoted by numerals 21-.

[0040] The light beam introduced into the interferometer is divided by a1:1 half mirror 21 into two light beams. The reflected light beam goesvia a mirror 22 and then it is expanded by a beam expander 23. Theexpansion magnification may generally be 10× or more. Because of theexpansion, the light beam can be considered as being an approximatelyidealistic plane wave. Thus, as a reference beam, it is directed to alight receiving surface of a CCD 28, via a half mirror 24 and acondensing system 27.

[0041] On the other hand, the light beam passed through the half mirror21 goes via a mirror 25 as a measurement beam, and it is reflected by ahalf mirror 24, by which it is combined with the reference beam. Thelight beam is then passed through the condensing system 27 and it isdirected onto the light receiving surface of the CCD 28. Here, it is tobe noted that, for fine adjustment of the interferometer, the mirror 25is mounted on a mechanism 26 by which tilt and parallel eccentricity canbe adjusted. The measurement beam described above interfere with thereference beam described above, by which the wavefront of the projectionoptical system 16 can be measured.

[0042] The correction of a system error in the measured wavefront, useof a fringe scan method for enhancement of measurement precision, use ofa spherical mirror of convex surface mirror type, and calculation ofaberrations of the projection optical system may be done in a similarway as the first embodiment. On the basis of the results ofmeasurements, a predetermined lens or lenses of the projection opticalsystem 16 may be displaced, by which the aberrations of the projectionoptical system can be adjusted and controlled into a desired state.

[0043]FIG. 4 is a schematic view of a fourth embodiment of the presentinvention. Like the first embodiment, this embodiment is directed to anexcimer laser stepper having an exposure wavelength of 248 nm, wherein aFizeau type interferometer is provided on the wafer side.

[0044] Denoted at 6 is a light source for the interferometer, from whicha light beam of 248 nm corresponding to the second harmonic of Ar laseris extracted. The laser beam goes via a mirror, a condensing system 7and a pinhole 8. Through an optical system 9, it is transformed into aparallel beam. The parallel light beam then goes via a half mirror 10and a mirror 11, and it enters an objective lens 32. The curvatureradius of the final face of the objective lens 32 on the wafer side ismade equal to the distance to an imaging plane 17 of the projectionoptical system 16 on its wafer side. Thus, reflection light from thatfinal face is directed, as a reference light, to a light receivingsurface of a CCD 28 via a mirror 31, the half mirror 10 and a condensingsystem 27.

[0045] On the other hand, the light beam passed through the objectivelens 32 is imaged upon a plane 17 corresponding to the wafer surface.Then, it is imaged again by the projection optical system 16 upon aplane 15 corresponding to the reticle pattern. There is a stage 34 onthe reticle side, on which a spherical mirror 33 is mounted. Thespherical mirror has a curvature radius which is made equal to thedistance from the imaging position 15 of the projection optical system,corresponding to the reticle surface. Thus, the light reflected by thespherical mirror 33 is collected again at the imaging position 15 of theprojection optical system, corresponding to the reticle surface, andthen it goes back through the projection optical system 16. Then, it isdirected to the light receiving surface of the CCD 28 via the objectivelens 32, the mirror 31, the half mirror 10 and the condensing system 27.The light beam passed through the projection optical system 16interferes with the reference beam as reflected by the final face of theobjective lens 32 as described above, such that the wavefront of theprojection optical system 16 can be measured.

[0046] Since the detection optical system is provided on the wafer side,by using the movability of the wafer stage in X and Y directions,measurement can be done with respect to plural points within the pictureplane of the exposure region. Thus, with the movement of the waferstage, the spherical mirror 33 on the reticle side can be moved by thestage 34 to a predetermined position. Therefore, in addition to thewavefront measurement with respect to the individual measurement points,various wavefront aberrations such as distortion and field curvature,for example, of the projection optical system can be detected, bycalculation, from the measurement data obtained in relation to theplural points.

[0047] The correction of a system error in the measured wavefront, useof a fringe scan method for enhancement of measurement precision, andcalculation of aberrations of the projection optical system may be donein a similar way as the first embodiment. Also, a modification of usinga spherical mirror of convex surface mirror type on the reticle side,may be made easily. However, in the case of this embodiment, the fringescan can be accomplished by actuating a PZT device inside the reticleside stage 34 to shift the mirror 33 in the optical axis direction by anamount of about the wavelength, to cause phase modulation of thewavefront. Alternatively, the fringe scan may be accomplished byactuating a PZT device inside the wafer stage 19 to move the objectivelens 32 in the optical axis direction by an amount of about thewavelength, to cause phase modulation of the wavefront.

[0048] On the basis of the results of measurements, a predetermined lensor lenses of the projection optical system 16 may be displaced, by whichthe aberrations of the projection optical system can be adjusted andcontrolled into a desired state.

[0049]FIG. 5 is a schematic view of a fifth embodiment of the presentinvention. Like the first embodiment, this embodiment is directed to anexcimer laser stepper having an exposure wavelength of 248 nm, wherein asingle-path type radial share interferometer is provided on the reticleside.

[0050] Denoted at 6 is a light source for the interferometer, from whicha light beam of 248 nm corresponding to the second harmonic of Ar laseris extracted. The laser beam goes via a mirror 11 and enters anobjective lens 13. The light beam passing through the objective lens 13is imaged at a position 17 corresponding to the wafer position, and thenit is imaged again by the projection optical system 16 at a position 15on the reticle side. The light thus imaged at the position 15 isadvances via the objective lens 13, the mirror 11 and a half mirror 10,and it is introduced into an interferometer having components denoted bynumerals 21-.

[0051] The light beam introduced into the interferometer is divided by a1:1 half mirror 21 into two light beams. The reflected light beam goesvia a mirror 22 and then it is expanded by a beam expander 23. Theexpansion magnification may generally be 10× or more. Because of theexpansion, the light beam can be considered as being an approximatelyidealistic plane wave. Thus, as a reference beam, it is directed to alight receiving surface of a CCD 28, via a half mirror 24 and acondensing system 27.

[0052] On the other hand, the light beam passed through the half mirror21 goes via a mirror 25 as a measurement beam, and it is reflected by ahalf mirror 24, by which it is combined with the reference beam. Thelight beam is then passed through the condensing system 27 and it isdirected onto the light receiving surface of the CCD 28. Here, it is tobe noted that, for fine adjustment of the interferometer, the mirror 25is mounted on a mechanism 26 by which tilt and parallel eccentricity canbe adjusted. The measurement beam described above interfere with thereference beam described above, by which the wavefront of the projectionoptical system 16 can be measured.

[0053] The correction of a system error in the measured wavefront aswell as calculation of aberrations of the projection optical system, forexample, may be done in a similar way as the first embodiment. On thebasis of the results of measurements, a predetermined lens or lenses ofthe projection optical system 16 may be displaced, by which theaberrations of the projection optical system can be adjusted andcontrolled into a desired state.

[0054] In a case of an i-line stepper, a basic wave of an argon laser ofa wavelength 363.8 nm may be used.

[0055] In the embodiments of the present invention describedhereinbefore, an interferometer for measurement of an opticalperformance of a projection optical system is mounted on a majorassembly of a projection exposure apparatus, by which the wavefrontmeasurement for the projection optical system can be performed on themain assembly of the projection exposure apparatus.

[0056] Executing the measurement of an optical characteristic of aprojection optical system, on the main assembly of a projection exposureapparatus, enables checking the state of the projection optical systemas the same is there. It is therefore possible to take any measures inaccordance with the state of the projection optical system.

[0057] More specifically, as an example, the aberration state of theprojection optical system can be corrected in accordance with the resultof measurement, or a judgment as to whether the operation should beinterrupted or not can be made promptly. As a result of it, the exposureprocess can be performed with the imaging performance of the projectionexposure apparatus held at a high level. This provides a large advantagein production of semiconductor devices.

[0058] While the invention has been described with reference to thestructures disclosed herein, it is not confined to the details set forthand this application is intended to cover such modifications or changesas may come within the purposes of the improvements or the scope of thefollowing claims.

What is claimed is:
 1. A projection exposure apparatus, comprising: anillumination optical system for illuminating a pattern formed on a firstobject, with light; a projection optical system for projecting thepattern of the first object, illuminated by said illumination opticalsystem, onto a second object for exposure of the same with the pattern;a main system including said illumination optical system and saidprojection optical system; and an interferometer for use in measurementof an optical characteristic of said projection optical system and beingmounted on said main system.
 2. An apparatus according to claim 1,wherein said interferometer includes a detection optical system forobserving a light beam from one of the first and second objects, saiddetection optical system being disposed outside an exposure light fluxof said projection optical system in an exposure process and being movedonto a light path of said projection optical system in a process formeasurement of wavefront aberration of said projection optical system.3. An apparatus according to claim 1, further comprising first andsecond light sources, wherein said first light source is used with saidillumination optical system for illumination of the pattern, and whereinsaid second light source is used with said interferometer.
 4. Anapparatus according to claim 3, wherein said interferometer is operableto perform measurement with respect to plural points within an exposureregion of said projection optical system.
 5. An apparatus according toclaim 4, wherein an aberration characteristic of said projection opticalsystem within the exposure region is detected on the basis ofmeasurements made with respect to said plural points.
 6. An apparatusaccording to claim 5, wherein a curvature of image field of saidprojection optical system is measured on the basis of measurements madewith respect to said plural points.
 7. An apparatus according to claim6, wherein the curvature of image filed of said projection opticalsystem is detected on the basis of (i) a coordinate position of saiddetection optical system of said interferometer with respect to anoptical axis direction, upon measurements of a wavefront with respect tosaid plural points, (ii) the wavefront as measured by saidinterferometer, and (iii) a coordinate position of a spherical surfacemirror, provided in said interferometer, with respect to the opticalaxis direction of said projection optical system.
 8. An apparatusaccording to claim 5, wherein light from said projection optical systemis reflected by one of a flat mirror and a wafer.
 9. A system accordingto claim 5, wherein distortion of said projection optical system ismeasured on the basis of measurements made with respect to said pluralpoints.
 10. An apparatus according to claim 9, wherein the distortion ofsaid projection optical system is detected on the basis of (i) acoordinate position of said detection optical system of saidinterferometer with respect to an optical axis direction, uponmeasurements of a wavefront with respect to said plural points, (ii) thewavefront as measured by said interferometer, and (iii) a coordinateposition of a spherical surface mirror, provided in said interferometer,with respect to the optical axis direction of said projection opticalsystem.
 11. An apparatus according to claim 1, wherein saidinterferometer is disposed on a side of said projection optical system,facing to the first object.
 12. An apparatus according to claim 11,wherein said interferometer includes a spherical surface mirror disposedadjacent to an image plane which is on one side of said projectionoptical system facing to the second object.
 13. An apparatus accordingto claim 12, wherein said spherical surface mirror is mounted on a stagefor the second object, being provided in said main system.
 14. Anapparatus according to claim 13, wherein said spherical surface mirroris made movable along an optical axis direction of said projectionoptical system, through moving means being provided for focus adjustmentof said projection optical system within said main system.
 15. Anapparatus according to claim 14, further comprising a detection opticalsystem having a TTR alignment scope with an objective lens, beingmounted on said main system.
 16. An apparatus according to claim 1,wherein said interferometer is disposed on one side of said projectionoptical system, facing to the second object.
 17. An apparatus accordingto claim 16, wherein said interferometer includes a spherical surfacemirror disposed adjacent to an image plane on one side of saidprojection optical system facing to the first object.
 18. An apparatusaccording to claim 1, wherein said interferometer comprises a Fizeautype interferometer.
 19. An apparatus according to claim 1, wherein saidinterferometer comprises a Twyman-Green type interferometer.
 20. Anapparatus according to claim 1, wherein said interferometer comprises aradial share type interferometer.
 21. fAn apparatus according to claim1, wherein measurement through said interferometer is performed inaccordance with a fringe scan method.
 22. An apparatus according toclaim 1, further comprising light guiding means for guiding light from alight source of said interferometer into a light path of said projectionoptical system.
 23. An apparatus according to claim 1, wherein saidinterferometer is used with light of double harmonic of an argon laserof a wavelength 496 nm.
 24. An apparatus according to claim 1, whereinsaid interferometer is used with light of a basic wave of an argon laserof a wavelength 368.8 nm.